(1) Field of the Invention
The present invention relates to an image forming apparatus, and especially relates to an image forming apparatus provided with a photosensitive drum which is driven by a stepping motor.
(2) Related Art
A conventional photosensitive drum is rotated in the sub-scanning direction and exposed by a laser beam in a certain cycle in the main scanning direction, and as a result, an electrostatic latent image is formed on the surface of the photosensitive drum. When the rotational speed of the photosensitive drum is unstable, distances between the scanning lines are not kept uniform, thereby causing deterioration on a reproduced image. As such, in recent years, a stepping motor is used as a driving source of the photosensitive drum for its high performance in braking in comparison with other kinds of motors.
The stepping motor can be made to rotate by a step angle which is smaller than a standard step angle, such as a step angle that is half of the standard step angle (this drive mode is referred to as the "half-step drive" hereinafter). Consequently, the photosensitive drum is rotated more smoothly.
However, when the half-step drive is operational, a rotor is alternately rotated with a salient pole facing a salient pole of a stator and with the salient pole being located between two salient poles of the stator, making precision of the step angle low. More specifically, when the salient pole of the rotor is located between the two salient poles of the stator, forces of the two salient poles of the stator to attract the salient pole of the rotor are not always equal, so that the salient pole of the rotor is attracted more strongly to one of the two salient poles of the stator.
Thus, if an electrostatic latent image formation is started when the salient pole of the rotor is located between the two salient poles of the stator, a starting position of the electrostatic latent image formation on the photosensitive drum is deviated from the correct starting position in the sub-scanning direction.